JP2000284613A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain a good transfer performance for each color in all the image forming parts while accomplishing the simplification of a transfer bias control and the miniaturization of a transfer power source, etc., by regulating the electric characteristics of an intermediate transfer body. SOLUTION: The image forming device is provided with plural images forming parts 10Y to 10K along the moving direction of the intermediate transfer belt 8, and each color toner image formed on each photoreceptor drum 70Y to 70K is successively primary-transferred to the surface of the belt 8, then, secondary-transferred on to a recording material P in a mass. Provided that a time required for the belt 8 to move between adjacent transfer positions is expressed by T (second), and the electrostatic charging mitigation time of the intermediate transfer body is expressed by τ (second), τ<=T is established. Thus, at the primary transfer process, all the primary transfer biases applied on primary transfer rollers 54Y to 54K coming into contact with the photoreceptor drums 70Y to 70K with the belt 8 interposed are made equal, besides, it is not necessary to change the primary transfer bias and the secondary transfer bias with the number of printed sheets, then, each transfer bias is simplified, and then a good full color image is stably obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a copying machine,
The present invention relates to an image forming apparatus such as a printer and a facsimile, and more particularly to an image forming apparatus that transfers an image on an image carrier to an intermediate transfer member and transfers an image on the intermediate transfer member to a recording material.

[0002]

2. Description of the Related Art Conventionally, an image forming apparatus for transferring a toner image formed on an image carrier using an electrophotographic method to a recording material and fixing the unfixed toner image to obtain a permanent image on the recording material. The demand for color image forming apparatuses is increasing with the progress of the information society in recent years.

FIG. 5 shows a schematic configuration of an example of a conventional electrophotographic full-color image forming apparatus. In this image forming apparatus, a plurality of photoconductors (image carriers) are stacked and a toner image is sequentially formed on each photoconductor in order to speed up color image output. After that, batch transfer to the recording material is performed.

As shown in FIG. 5, the present image forming apparatus comprises four color image forming units (image forming stations) 10Y, 10M, 10C, yellow, magenta, cyan, and black.
10K, an intermediate transfer belt 80 as an intermediate transfer member, and a fixing device 40 as a fixing unit.

The image forming units 10Y, 10M, 10C, 1
Reference numeral 0K denotes a unit, and the photosensitive drums 70Y, 70M, 70C, and 70K, which are image carriers, and primary charging rollers 12Y, 12M, 12C, and 12K are provided around the photosensitive drums.
K, laser exposure units 13Y, 13M, 13C, 13K,
Developing units 14Y, 14M, 14C, 14K, primary transfer rollers 54Y, 54M, 54C, 54K, cleaner 16Y,
16M, 16C and 16K are provided. The intermediate transfer belt 80 is disposed in contact with each of the photosensitive drums 70Y to 70K, is stretched around three rollers of a driving roller 51, a tension roller 52, and a secondary transfer opposing roller 53, and is arranged in a direction indicated by an arrow b in FIG. It is driven to rotate.

The respective photosensitive drums 70 (70Y to 70K)
After the surface is uniformly charged by the primary charging roller 12 (12Y to 12K), the laser exposure device 13 (13Y to 13K) is used.
Image exposure is performed by color separation in K), and an electrostatic latent image corresponding to the document is formed on the surface of the photosensitive drum 70. This latent image is developed using negative toner by the developing devices 14 (14Y to 14K), and a toner image is formed on the surface of the photosensitive drum 70.

In each of the image forming units 10Y to 10K, the above-described image forming operation is performed at a predetermined timing, whereby a toner image of each color is formed on the photosensitive drum 70. The primary transfer roller 5
4 (54Y to 54K), and sequentially transfers (primary transfer) onto the intermediate transfer belt 80 at each primary transfer section facing each other to form a full-color image on which toner images of four colors of yellow, magenta, cyan, and black are superimposed. Formed once on the intermediate transfer belt 80.

Thereafter, the four color toner images are fed to the paper feed roller 2.
0 on the recording material P supplied at a predetermined timing.
The images are collectively transferred using the secondary transfer roller 55 (secondary transfer). The recording material P that has completed the transfer process is conveyed to the fixing device 40, where it is heated and pressed to fix the toner image.

In the full-color image forming apparatus of the intermediate transfer body type, as described above, the four color toner images on the intermediate transfer body are collectively transferred onto a recording material, so that color misregistration (color registration) is small. Are better. According to the intermediate transfer member system, for example, the recording material is adsorbed and conveyed to a recording material carrier such as a transfer belt or a transfer drum, and the toner images of each color formed on the photosensitive drum are directly transferred to the recording material, and the recording is performed. Unlike the method of superimposing toner images of each color on a material,
There is no need to adsorb and transport the recording material. For example, a full-color toner image formed by rotating an intermediate transfer body such as an intermediate transfer belt is collectively transferred to the recording material. , And the color registration does not change due to the thickness of the recording material.

Therefore, among these electrophotographic full-color image forming apparatuses, an image forming apparatus using an intermediate transfer member is widely used.

[0011]

However, such a primary transfer system usually requires complicated transfer bias control. In order to achieve good transferability in all the image forming units 10Y to 10K, the constant voltage bias is set to be increased toward the downstream image forming unit so that a sufficient transfer current can be obtained in all the image forming units. It is necessary to apply the transfer bias independently from each image forming unit by a total of four high-voltage power supplies.

[0012] This is because the intermediate transfer belt, which sequentially passes through the image forming sections and superimposes and transfers the toner images of the respective colors, is gradually charged up, and the effective thickness in the thickness direction of the intermediate transfer belt passing through the transfer nip section is increased. Characteristic impedance is
This is because it becomes higher as it passes through the downstream image forming unit.

SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus capable of forming an image on an intermediate transfer member without charging up the intermediate transfer member.

[0014] Other objects of the present invention will become apparent upon reading the following detailed description.

[0015]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides:
A plurality of image carriers for carrying images; and an intermediate transfer member on which the plurality of images on the plurality of image carriers are electrostatically sequentially transferred at respective transfer positions. When transferring a plurality of images from the plurality of image carriers to the intermediate transfer body in an image forming apparatus in which the image is transferred to a recording material, the time required for the intermediate transfer body to move between adjacent transfer positions is T.
(Seconds), where τ ≦ T is satisfied, where τ (seconds) is the charge relaxation time of the intermediate transfer member.

In the present invention, the charge relaxation time τ is
When the potential of the intermediate transfer body charged to the potential V by the charging means is V / e (e is a natural logarithm base, e = 2.771827)
...). When the charging unit charges the intermediate transfer member to a potential V, a voltage obtained by superimposing a DC voltage and a high voltage is applied to the charging unit.

A plurality of images are provided on the side opposite to the side on which the image of the intermediate transfer body is transferred, and a plurality of images are respectively electrostatically transferred from the plurality of image carriers to the intermediate transfer body at the respective transfer positions. The image forming apparatus includes a plurality of transfer units, and the DC voltage is substantially equal to an absolute value of a difference between a potential of the exposed image carrier and a voltage applied to the transfer unit during image transfer. During image transfer, substantially the same voltage is applied to each of the transfer units. At the time of image transfer, the voltage applied to each transfer unit is controlled at a constant voltage. It has a plurality of power supplies for applying a voltage to each of the transfer means at the time of image transfer. It has a single power supply for applying a voltage to each transfer means.

The time T is the time required for the intermediate transfer member to move the distance between adjacent transfer positions. From the position where the image on the intermediate transfer member is transferred to the recording material to the position where the image is first transferred to the intermediate transfer member, the time required for the intermediate transfer member to move is defined as T ′ (second). τ ≦ T ′ holds. From the position where the last image is transferred to the intermediate transfer member,
Assuming that the time required for the intermediate transfer member to move to the position where the image on the intermediate transfer member is transferred to the recording material is T ″ (second), τ ≦ T ″ is satisfied.

The intermediate transfer member is a belt supported by a plurality of supporting means. A plurality of color images are sequentially transferred from the plurality of image carriers to the intermediate transfer member in a superimposed manner, and the plurality of color images on the intermediate transfer member are transferred to a recording material.

[0020]

Embodiments of the present invention will be described below in more detail with reference to the drawings.

Embodiment 1 FIG. 1 is a schematic structural view showing an embodiment of the image forming apparatus of the present invention. The image forming apparatus is configured as an intermediate transfer type full-color printer using four photosensitive drums.

As shown in FIG. 1, the present image forming apparatus
Color image forming units (image forming stations), that is, yellow (Y), magenta (M), cyan (C), and black (K) image forming units 10Y, 10M, 10C, and 10K
And an intermediate transfer belt 8 as an intermediate transfer member, and a fixing device 40 as a fixing unit.

Each of the image forming units 10Y, 10M, 10C, 1
Reference numeral 0K is a unit, and the photosensitive drums 70Y, 70M, 70C, and 70K, which are image carriers, are installed so as to be rotatable in the direction of arrow a. The photosensitive drums 70Y, 7
0M, 70C and 70K on the outer periphery of the primary charging roller 12
Y, 12M, 12C, and 12K are provided with laser exposure units 13Y, 13M, 13C, 1
3K is arranged, and the laser exposure units 13Y to 13K expose the photosensitive drums 70Y to 70K with laser light modulated in accordance with image signals emitted from the laser exposure units 13Y to 13K. Further downstream, developing units 14Y, 14M, 14C and 14K are arranged, and contain yellow, magenta, cyan and black toners, respectively.

Each photosensitive drum 70Y, 70M, 70C, 7
Primary transfer rollers 54Y, 54M, 54C and 54K are arranged with respect to 0K with the intermediate transfer belt 8 interposed therebetween, and high-voltage power supplies (constant-voltage power supplies) 48Y, 48M, 48C and 48 are respectively provided.
K applies primary transfer biases Vy, Vm, Vc, and Vk.

The intermediate transfer belt 8 is disposed so as to contact the photosensitive drums 70Y to 70K of the image forming units 10Y to 10K, and includes a driving roller 52, a tension roller 5
It is stretched over three rollers of the primary and secondary transfer opposing rollers 53 and is driven to rotate in the direction of arrow b in the figure.

In the mono color mode (for example, when a black and white image is formed) or the like, the intermediate transfer belt 8 may be swung and separated so as to contact only the desired photosensitive drum. Further, in the standby mode in which no image forming signal is input, the configuration may be such that the intermediate transfer belt 8 is separated from all the photosensitive drums.

Further, each of the photosensitive drums 70Y, 70M, 7
On the downstream side of 0C and 70K, the cleaners 16Y, 16M,
The intermediate transfer belt 8 is provided with a belt cleaner 33 at a tension roller 51.

The operation of the above image forming apparatus will be described by taking the yellow image forming unit 10Y as an example.

The photosensitive drum 70Y has a photo-semiconductive layer on the surface of the aluminum cylinder, and after the surface is uniformly negatively charged to about -500 V by the primary charging roller 12Y in the process of rotating in the direction of arrow a, Image exposure is performed by the laser exposure unit 13Y, and an electrostatic latent image corresponding to the document is formed on the surface of the photosensitive drum 70Y in a bright portion (laser exposure portion potential: -2).
00V) and a dark portion (non-exposed portion potential: -500 V). This latent image is developed using a negatively charged yellow toner by the developing device 14Y, and the photosensitive drum 70Y
A yellow toner image is formed on the surface. Photosensitive drum 7
The yellow toner image formed on 0Y is transferred onto the intermediate transfer belt 8 by the primary transfer roller 54Y (1
Next transfer). After the transfer, the residual toner remaining on the surface of the photosensitive drum 70Y is removed by the cleaner 16Y, and is then subjected to the next image formation.

The above operation is performed at predetermined timing in each of the image forming units 10Y to 10K, and each of the photosensitive drums 70Y to 70K and the primary transfer rollers 54Y to 54K.
K and the primary transfer portions T1, T2, T3, and T4,
The toner images of the respective colors are sequentially transferred onto the intermediate transfer belt 8 in a superimposed manner.

In the case of the full color mode, the toner images are transferred to the intermediate transfer belt 8 in the order of yellow, magenta, cyan, and black. Are transferred in the same order as.

Thereafter, the four-color toner images are transferred to the secondary transfer roller 5 as the intermediate transfer belt 8 rotates in the direction of arrow b.
5 and a secondary transfer opposing roller 53 that is grounded, is moved to a secondary transfer section, and a high voltage power supply (constant voltage power supply) 49 is provided on the recording material P supplied at a predetermined timing by the paper feed roller 20. Are transferred collectively by the secondary transfer roller 55 to which the secondary transfer bias W is applied (secondary transfer).
The surface of the intermediate transfer belt 8 on which the secondary transfer has been completed is cleaned by the belt cleaner 33.

In this embodiment, the photosensitive drums 70Y to 70K
Uses a negatively charged OPC drum with a diameter of 30.6 mm,
A charging bias in which an AC component is superimposed on a DC component is applied to the charging rollers 12Y to 12K, so that the photosensitive drums 70Y to 70K are uniformly charged to about -550 V regardless of the environment. The exposure units 13Y to 13K have a wavelength of 760 nm.
Near infrared laser diode and photosensitive drums 70Y-7
A polygon scanner that scans a laser beam on 0K.

Yellow developing device 14Y, magenta developing device 1
4M, cyan developing device 14C and black developing device 14K
Is a developing device of a jumping development system using a non-magnetic one-component toner, and a core / wax containing a wax as a toner.
A negatively chargeable polymer toner having a particle diameter of 6 μm having a shell structure is applied on the developing sleeve, and the thickness of the toner layer is regulated by an elastic blade.
C, Jump to the electrostatic latent image on 70K and perform reversal development.

Each of the primary transfer rollers 54Y-54K, the secondary transfer roller 55, and the secondary transfer opposing roller 53 are formed by applying a conductive rubber layer having a volume resistivity of 1 × 10 ⁵ Ωcm to a core having a diameter of 14 mm in the longitudinal direction. Each roller is coated so that the diameter of each roller becomes 20 mm over 310 mm. The primary transfer rollers 54Y to 54K have respective metal cores connected to high voltage power supplies 48Y to 48K via power supply springs, and the secondary transfer roller 55 has a high voltage power supply 49 connected to the secondary transfer facing roller 53.
Is connected to ground.

In this embodiment, the distance between two adjacent photosensitive drums (between two adjacent primary transfer portions) is determined by the circumference of the drive roller 52 (the thickness of the intermediate transfer belt is the radius of the drive roller 52). If it is not negligible as compared with the above, it is preferable to set the circumferential length in consideration of the thickness). Not limited to such a configuration,
The distance between the next transfer units may be an integral multiple of the circumference of the driving roller. With such a configuration, it is possible to prevent color shift due to uneven speed of the intermediate transfer belt due to eccentricity of the driving roller.

The intermediate transfer belt 8 has a circumference of 1115 m.
m, length in the width direction (length in the same direction as the longitudinal direction of the photosensitive drum)
In the present invention, the intermediate transfer belt 8
It is a great feature that a material having a self-damping type electrical characteristic is used as the material.

Here, in the present invention, the self-decay type means that the charging relaxation time of the intermediate transfer member is τ (second), and the interval between two adjacent image bearing members (between two adjacent primary transfer portions (T1 to T2) , T
2 to T3, T3 to T4)), when the time required for a part of the intermediate transfer member to move is defined as T (seconds), it means that a characteristic that satisfies the relationship of τ ≦ T is satisfied. on the other hand,
Those that do not satisfy this condition are called charge-up types.

The charge relaxation time τ of the intermediate transfer belt 8 is
The potential V applied at the charging position of the intermediate transfer belt is V / e (e is a natural logarithm base, e = 2.718...)
It is defined as the time until it decreases.

The charge relaxation time τ is measured by using the apparatus shown in FIG. That is, since the value obtained by simply multiplying the capacitance and the resistance of the intermediate transfer belt 8 does not match, in the present invention, the apparatus shown in FIG.
The time measured by the method is defined as “τ”.

The intermediate transfer belt 8 is stretched around a drive roller 207 as a measuring jig and a metal tension roller 206, and rotates in a direction indicated by an arrow at a speed of 117 mm / sec. The intermediate transfer belt 8 is moved to the charging roller 20 at the charging position.
1. It is sandwiched between metal opposing rollers 208 and charged by an AC power supply 202 with a peak-to-peak voltage Vpp of about 3 kV and a DC power supply 203 of +500 V.

The environment during the measurement was a temperature of 23 ° C. and a relative humidity of 6
0%. The voltage applied to the charging roller 201 is
The voltage corresponds to the absolute value of the difference between the bias of 300 V applied to the primary transfer roller during normal image formation in the above environment and the light potential of the photosensitive drum of about -200 V.

Further, in this embodiment, by applying a voltage obtained by superimposing a DC voltage and an AC voltage to the charging roller 201, the intermediate transfer belt 8 in contact with the charging roller 201 is applied.
Is charged to a potential substantially equal to the DC voltage (500 V). The Vpp and frequency of the AC voltage may be set as appropriate.

The charging roller 201 is of a well-known contact charging type. For example, a medium resistance layer having a volume resistivity of about 10 ⁶ Ωcm and a medium resistance layer of about 100 to 200 μm is provided on an elastic conductive rubber having a thickness of about 3 mm. It has a cylindrical shape with a diameter of about 12 mm provided with an anti-sticking layer (nylon resin or the like) of 10 μm.

The surface potential W of the intermediate transfer belt 8 charged by the charging roller 201 is measured by a surface voltmeter probe 204 and a voltmeter main body 205 provided at a position rotated for T seconds downstream from the charging position. T is 0.8 of the time required for a part of the intermediate transfer belt to move between two adjacent image carriers of the image forming apparatus of the present embodiment.
It is the same time as seconds.

At this time, it can be said that the intermediate transfer belt 8 is a self-damping type if W ≦ 500 / e [V], and a charge-up type if W> 500 / e [V].

In this embodiment, two types of intermediate transfer belts, charge-up type A and self-attenuation type B, were prepared and used in experiments to examine the characteristics in image formation. Hereinafter, this will be described.

The intermediate transfer belt A has a three-layer structure including a surface layer, an intermediate layer, and a base layer. The surface layer has a volume resistivity of 1 × 10 ¹⁶
It was formed of a urethane resin having Ωcm, a thickness of 10 μm, and PTFE, a fluororesin having good releasability, dispersed therein. The intermediate layer has a volume resistivity of 1 × 10 ¹⁰ Ωcm and a thickness of 10 μm, and the base layer has a volume resistivity of 1 × 10 ⁷ Ωcm and a thickness of 8 μm.
It was formed of a rubber mainly composed of NBR / EPDM mixed rubber.

The intermediate transfer belt B has a two-layer structure of a surface layer and a base layer. The surface layer has a volume resistivity of 1 × 10 ¹² Ωcm, a thickness of 20 μm, and is formed of a medium-resistance urethane resin in which an aggregate is dispersed. The base layer had a volume resistivity of 1 × 10 ⁶ Ωcm and a thickness of 1000 μm, and was formed of a rubber mainly composed of NBR / epichlorohydrin mixed rubber.

In the image forming apparatus of this embodiment, the maximum size that can be used as the recording material P is A3 size.
The process speed is 117 mm / sec.

The above-mentioned intermediate transfer belts A and B are mounted on the image forming apparatus shown in FIG. 1, and the primary transfer rollers 54Y are used so that the primary transfer efficiency of each color is maximum and a good full-color image is obtained. , 54M, 54C, and 54K, the optimum values of the primary transfer biases Vy, Vm, Vc, and Vk were obtained. The results shown in Table 1 were obtained.

[0052]

[Table 1] As shown in Table 1, when the intermediate transfer belt A is used, the primary transfer bias is not applied to each color by applying a higher transfer bias toward the downstream image forming section such as Vy <Vm <Vc <Vk. It can be seen that the primary transfer bias can be optimized. This is because each image forming unit 10
Y, 10M, 10C, and 10K, the electric charge is gradually charged inside the intermediate transfer belt, and is electrically charged up. Therefore, the effective impedance in the thickness direction of the intermediate transfer belt that passes through the transfer nip portion is downstream. This is because it becomes higher as it passes through the image forming unit on the side.

Therefore, all image forming units 10Y to 10K
In order to achieve good transferability, the constant voltage bias must be set higher in the downstream image forming section so that a sufficient transfer current can be obtained in all image forming sections. In addition, the results in Table 1 show that the charge-up of the intermediate transfer belt further proceeds, so that the values of the primary transfer biases Vy, Vm, Vc, and Vk must be further increased in accordance with the number of continuous prints. Indicates that it must not.

Since the intermediate transfer belt is charged up, the secondary transfer bias W applied to the secondary transfer roller 55 using the secondary transfer bias high voltage power supply 49 is variable according to the type of the recording material P. In addition, even when the recording material P is the same during continuous printing, the bias W must be set so as to be sequentially increased.

Therefore, when a charge-up type belt is used as the intermediate transfer belt 8, the control of the primary transfer bias and the control of the secondary transfer bias become complicated, and it is difficult to always obtain a good full-color image stably.

On the other hand, when the intermediate transfer belt B is used, the primary transfer bias is Vy = Vm = Vc = Vk, and it can be seen that the primary transfer bias may be the same in all the image forming units 10Y to 10K. . This is because the intermediate transfer belt B is of a so-called electrically self-decaying type having a short relaxation time of the charges charged inside the belt and having no charge-up characteristic. However, the effective impedance in the belt thickness direction at the transfer nip portion of the intermediate transfer belt remains in the initial state before passing through the yellow image forming unit 10Y, and the good transferability of each color in all image forming units is substantially improved. This is because they can be obtained with the same primary transfer bias value. This primary transfer bias value is always kept constant even during continuous printing.

A secondary transfer bias W applied to the secondary transfer roller 55 using the secondary transfer bias high voltage power supply 49.
Also, it is not necessary to sequentially raise the recording material P during continuous printing, and it is only necessary to make it variable according to the type of the recording material P.

Therefore, if a self-attenuating type is used as the intermediate transfer belt 8, there is no need to separately provide a device for initializing (eliminating) the potential of the intermediate transfer belt 8 after the secondary transfer. Transfer bias control and 2
The control of the next transfer bias is simplified, and a good full-color image can be stably obtained.

As described above, as a result of the detailed examination of this embodiment, it is found that the primary transfer bias control and the secondary transfer bias control of each color can be performed by using a self-attenuating type as the intermediate transfer belt 8 provided in the image forming apparatus. Simplification is achieved, and a good full-color image can be stably obtained.

In this embodiment, assuming that the time required for the intermediate transfer belt 8 to move from the secondary transfer section to the first (most upstream) primary transfer section T1 is T '(seconds), the relationship τ≤T' holds. ing. Therefore, in this embodiment, 2
After the next transfer and before the primary transfer, a special static eliminator for eliminating and initializing the intermediate transfer belt becomes unnecessary, and the size and cost of the image forming apparatus can be further reduced. Similarly, the time required for the intermediate transfer belt to move from the last (most downstream) primary transfer portion T4 to the secondary transfer portion is represented by T ″ (second).
Then, the relationship of τ ≦ T ”also holds.

In this embodiment, when the single color, two color, or three color mode is selected, the electrical deterioration of the photosensitive drum,
In order to prevent mechanical deterioration of the photosensitive drum due to friction with the intermediate transfer belt, the photosensitive drum on which no image is formed and the intermediate transfer belt may be appropriately separated.

Note that a constant current power supply may be used as a high voltage power supply for the primary transfer and the secondary transfer. Even with such a configuration, the voltage applied from the power supply to the primary transfer roller and the secondary transfer roller can be reduced, and control can be easily performed.

In the above, the roller-like primary transfer roller 54
Although the secondary transfer roller 55 is used for Y to 54K, it can be replaced with a blade-like or brush-like one, and the present invention can be similarly applied.

Embodiment 2 FIG. 2 is a schematic diagram showing another embodiment of the image forming apparatus of the present invention.

In this embodiment, the self-attenuation type intermediate transfer belt B shown in Embodiment 1 is used as the intermediate transfer belt 8.
Further, the high-voltage power supply for controlling the primary transfer bias has been simplified.
Other configurations of the image forming apparatus according to the present embodiment are basically the same as those of the image forming apparatus according to the first embodiment, and a detailed description thereof will be omitted.

In this embodiment, the primary transfer rollers 54Y and 54Y of each image forming section (image forming station) of the image forming apparatus are used.
54M, 54C and 54K have one common high-voltage power supply 4
7, the same transfer bias Z = 300 V is applied in parallel.
At the same time.

A secondary transfer bias X that is variable according to the type of the recording material P is applied to the secondary transfer roller 55 using the high voltage plate 49 for the secondary transfer bias. However, the secondary transfer bias X has a relationship of X = W + Z with W applied by the high voltage power supply 49 in the image forming apparatus of the first embodiment.

The primary transfer high-voltage power supply 47 in this embodiment is reduced in size and cost. This is because the intermediate transfer belt 8 is of a self-attenuation type, and it is not necessary to change the primary transfer bias Z and the secondary transfer bias X according to the number of continuous prints. In this embodiment,
With the simplified bias control as described above, a good full-color image can be stably obtained.

Since the primary transfer rollers 54Y-54K and even the secondary transfer opposing roller 53 all have the same potential, unnecessary leak current generated between the rollers through the inner surface of the intermediate transfer belt 8 is prevented. . It is preferable that the resistance of the back surface of the intermediate transfer belt 8 is low. Therefore,
The power consumption of the high-voltage power supply 47 can be kept low.
Also, by controlling the primary transfer bias applied to the primary transfer rollers 54Y to 54K and the bias applied to the secondary transfer opposing roller 53 to be always on and off at the same timing, the electricity between the transfer units (between the rollers) is controlled. Image defects due to mechanical interference are also suppressed.

Further, the tension roller 51,
Similarly, Z = 300 V may be applied to the drive roller 52. With such a configuration, it is possible to prevent an image defect (due to insufficient transfer current) due to electrical interference (leak) between the rollers (primary transfer roller, secondary transfer opposing roller, drive roller, tension roller). be able to.

As described above, in this embodiment, the same bias is applied in parallel to the primary transfer roller of each image forming unit by using one high-voltage power supply, using the self-attenuating intermediate transfer belt. The high voltage power supply for the primary transfer can be reduced in size and cost, and by applying the same bias to the secondary transfer opposing roller, power consumption can be reduced by suppressing leakage current.

In the first and second embodiments, a rubber belt having a multi-layer structure is described as the intermediate transfer belt. However, a similar effect can be obtained with a single-layer belt or a belt made of resin or the like.

Embodiment 3 FIG. 3 is a schematic structural view showing still another embodiment of the image forming apparatus of the present invention.

In the present embodiment, an intermediate transfer drum 91 is used as shown in FIG. 3 instead of the intermediate transfer belt 8 of the embodiment 1 shown in FIG. , Magenta, cyan, and black image forming units 10Y, 10M, 10C, and 10K.

The image forming units 10Y, 10M, 10C, 10
K is the laser exposure device 13Y, 13M, 13 of the first embodiment.
Instead of C and 13K, LED exposure units 90Y and 90M,
90C and 90K were used. Other configurations of the present embodiment are basically the same as those of the first embodiment. In FIG. 3, the same reference numerals as those shown in FIG. 1 denote the same members.

In the present embodiment, similarly to the first embodiment, a toner image is formed on the surface of each of the photosensitive drums 70Y, 70M, 70C, and 70K at a predetermined timing. To 70K and the intermediate transfer drum 91, the primary transfer portions T1, T
At 2, T3 and T4, multiple transfer is sequentially performed on the intermediate transfer drum 91.

According to the present embodiment, the intermediate transfer drum 91 has a diameter of 186 mm and a width (length in the axial direction) of 310 mm, and a conductive rubber layer having a thickness of 5 mm is formed on an aluminum core bar. Is coated with a surface layer having a thickness of 20 μm, and has a so-called solid drum shape. The conductive rubber layer is a rubber mainly composed of NBR / epichlorohydrin mixed rubber and has a volume resistivity of 1 × 10 ⁶ Ω.
cm. The surface layer is made of a medium resistance urethane resin in which a lubricant is dispersed, and has a volume resistivity of 1 × 10 ¹² Ωcm. A 300 V is supplied to the aluminum core of the intermediate transfer drum 91 by a high-voltage power supply 47 through a power supply spring (not shown).
Is applied.

The toner images of the respective colors, which have been primarily transferred on the intermediate transfer drum 91, are brought into contact with the intermediate transfer drum 91 to form a secondary transfer nip portion by a secondary transfer device 95.
It is electrostatically and collectively transferred onto the recording material P conveyed at a predetermined timing (secondary transfer).

In the present embodiment, the secondary transfer device 95 is configured by winding a secondary transfer belt 92 around a secondary transfer roller 93 and a driving roller 94. Secondary transfer device 95
Is a secondary transfer roller 93 on the upstream side in the conveying direction of the recording material P.
Are arranged so as to contact the intermediate transfer drum 91 via the secondary transfer belt 92. The drive roller 94 drives the secondary transfer belt 92 to rotate in the direction of arrow c so that the peripheral speed of the intermediate transfer drum 91 is equal to the peripheral speed of the secondary transfer belt 92.

The secondary transfer device 95 is installed so as to be able to freely contact and separate from the intermediate transfer drum 91, and comes into contact with the intermediate transfer drum 91 via the recording material P during the secondary transfer. The contact pressure is 3.2 kgf. Further, in order to electrostatically transfer the toner image from the intermediate transfer drum 91 onto the recording material P, the core metal portion of the secondary transfer roller 93 is variable from the high voltage power supply 49 depending on the type of the recording material P. The secondary transfer bias W is applied. Due to the application of the secondary transfer bias, a secondary transfer current flows from the secondary transfer roller 93 toward the intermediate transfer drum 91, and charges are supplied from the secondary transfer belt 92 in the direction of the recording material P, thereby causing the secondary transfer current to flow on the intermediate transfer drum 91. Is secondary-transferred onto the recording material P.

The secondary transfer roller 93 and the secondary transfer belt driving roller 94 are each formed by forming a conductive rubber layer having a volume resistivity of 1 × 10 ⁵ Ωcm on a metal core having a diameter of 14 mm in a longitudinal direction of 3 mm.
This is a roller coated so as to have a diameter of 20 mm over 10 mm. The core of the secondary transfer roller 93 is connected to a high-voltage power supply via a power supply spring.
2 and rotate. Secondary transfer belt drive roller 94
Is driven by transmitting a driving force from a driving mechanism (not shown).

The secondary transfer belt 92 is a seamless belt having a width of 310 mm and an inner diameter of 65 mm.
It is stretched and stretched by 5% between the secondary transfer roller 93 and the secondary transfer belt driving roller 94 arranged at 5 mm. The thickness of the secondary transfer belt 92 is 310 μm, and a surface layer of PTFE-based rubber having a thickness of 20 μm and a thickness of 29 μm are formed.
And a base layer of 0 μm carbon dispersed elastomer. The volume resistivity of the base layer is 1 × 10 ⁶ Ωcm, and the surface resistivity on the surface layer side of the transfer belt is 1 × 10 ¹² Ωcm.

Here, the secondary transfer device 95 electrostatically transfers the toner image onto the recording material P, and simultaneously
The recording material P is separated from the surface of the intermediate transfer drum 91 by being electrostatically attracted to the next transfer belt 92. Note that the secondary transfer device 95 can be configured using a single transfer roller.

As described above, the recording material P on which the toner image has been transferred is conveyed to the fixing device 40, and the toner image is permanently fixed on the recording material P by heat and pressure. It is discharged outside the forming device. The secondary transfer residual toner remaining on the surface of the intermediate transfer drum 91 after the completion of the secondary transfer is removed and collected by the drum cleaner 96.

The image forming apparatus of this embodiment has a diameter of 186 m.
Since there are m intermediate transfer drums 91, the maximum sheet passing size that can be used as the recording material P is A3 size. The process speed of the image forming apparatus is 117 m.
m / sec.

The primary transfer bias in this embodiment is 30.
0 V, which is applied to the cylindrical aluminum core of the intermediate transfer drum 91 via a power supply spring, so that a primary transfer bias of 300 V is uniformly applied to all the primary transfer portions. become. Since the intermediate transfer drum 91 used in the present embodiment is also of an electrically self-attenuating type, good transferability in all image forming units is always obtained by setting the bias, and a good full-color image can be stably obtained. it can.

That is, also in this embodiment, since τ ≦ T is satisfied, the residual potential of the intermediate transfer drum is sufficiently reduced before reaching the next primary transfer portion of the intermediate transfer drum 91, and In a stable state, the next primary transfer can be performed satisfactorily.

At the time of the primary transfer, a contrast between the potential of the light portion (potential: VL) and the potential of the dark portion (potential: VD) on the photosensitive drum is formed on the surface of the intermediate transfer drum. By the time the intermediate transfer belt arrives at the next primary transfer section, the potential contrast is lost and initialization is performed, and a halftone image can be favorably formed.

The determination as to whether or not the intermediate transfer drum is a self-attenuation type can be made by the same apparatus as the apparatus shown in FIG.

Further, in the image forming apparatus of this embodiment, an LED exposing unit is used instead of the laser exposing unit, and an intermediate transfer drum is used as an intermediate transfer body. Registration is further improved. LED exposure equipment, compared to laser exposure equipment,
The deviation of the image in the main scanning direction can be reduced, and the color registration in the main scanning direction is excellent. Further, the LED exposure device also contributes to miniaturization of each of the image forming units 10Y to 10K.

Further, in general, the intermediate transfer method has an advantage that the color registration hardly occurs due to the thickness of the recording material. However, the use of the intermediate transfer drum can further improve this.

As described above, in this embodiment, the self-attenuating type intermediate transfer drum is used as the intermediate transfer body, and the LED exposing device is used as the exposing device. This makes it possible to obtain a full-color image with better resolution.

[0093]

As described above, the present invention relates to an image forming apparatus having a plurality of image carriers and an intermediate transfer member to which a plurality of images on the plurality of image carriers are sequentially transferred. When transferring a plurality of images from the image carrier to the intermediate transfer member,
The time required for the intermediate transfer member to move between adjacent transfer positions is T
(Seconds), assuming that the charge relaxation time of the intermediate transfer member is τ (seconds), τ ≦ T is established. Therefore, when the toner images of each color on a plurality of image carriers are sequentially primary-transferred onto the intermediate transfer member, the primary transfer bias can be all the same, and the primary transfer bias and the secondary transfer bias can be set. Need not be changed according to the number of prints, each transfer bias is simplified, and a good full-color image can be stably obtained.

In the case of the intermediate transfer belt, the primary transfer high-voltage power supply is reduced in size and cost by applying a primary transfer bias to a plurality of primary transfer means in parallel with one high-voltage power supply. Further, when a plurality of toner images on the intermediate transfer member are collectively and secondarily transferred to the transfer material, the same bias value is applied in parallel to the secondary transfer facing roller contacting the intermediate transfer member by the high voltage power supply. By doing so, it is possible to easily prevent leakage current and suppress power consumption. When an LED is used as an image exposure unit for a plurality of image carriers arranged in parallel with the intermediate transfer body, the size of the image forming unit is reduced, and a full-color image with more excellent color registration can be obtained. become.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing one embodiment of an image forming apparatus of the present invention.

FIG. 2 is a schematic configuration diagram showing another embodiment of the image forming apparatus of the present invention.

FIG. 3 is a schematic configuration diagram showing still another embodiment of the image forming apparatus of the present invention.

FIG. 4 is a diagram showing a measurement system for measuring a charge relaxation time of an intermediate transfer member.

FIG. 5 is a schematic configuration diagram illustrating a conventional image forming apparatus.

[Explanation of symbols]

 Reference Signs List 8 Intermediate transfer belt 12Y to 12K Charging roller 14Y to 14K Developing device 47 Primary transfer high voltage power supply 49 Secondary transfer high voltage power supply 53 Secondary transfer facing roller 54Y to 54K Primary transfer roller 55 Secondary transfer roller 70Y to 70K Photosensitive Drum 91 Intermediate transfer drum

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Tomoaki Nakai 3-30-2 Shimomaruko, Ota-ku, Tokyo F-term in Canon Inc. (reference) 2H030 AB02 AD16 BB42 BB46 BB54 2H032 AA05 AA15 BA01 BA07 BA23

Claims (14)

[Claims] 1. An image forming apparatus comprising: a plurality of image carriers for carrying images; and an intermediate transfer body on which a plurality of images on the plurality of image carriers are electrostatically sequentially transferred at respective transfer positions. In an image forming apparatus in which a plurality of images on a transfer body are transferred to a recording material, when transferring a plurality of images from the plurality of image carriers to the intermediate transfer body, the intermediate transfer body may move between adjacent transfer positions. An image forming apparatus, wherein τ ≦ T is satisfied, where T (second) is a moving time and τ (second) is a charge relaxation time of the intermediate transfer member. 2. The charging relaxation time τ is determined as follows: the potential of the intermediate transfer member charged to a potential V by a charging unit is V / e (e
2. The image forming apparatus according to claim 1, wherein ## EQU2 ## is the time until the value e falls to 2.71827. 3. The image forming apparatus according to claim 2, wherein when the charging unit charges the intermediate transfer member to a potential V, a voltage obtained by superimposing a DC voltage and a high voltage is applied to the charging unit. 4. A plurality of images are provided on a side opposite to a side on which an image of the intermediate transfer body is transferred, and a plurality of images are respectively electrostatically transferred from the plurality of image carriers to the intermediate transfer body at the respective transfer positions. A plurality of transfer means for transferring, wherein the DC voltage is substantially equal to an absolute value of a difference between an exposed potential of the image carrier and a voltage applied to the transfer means during image transfer. Item 3. The image forming apparatus according to Item 3. 5. The image forming apparatus according to claim 1, wherein T is a time during which the intermediate transfer body moves a distance between adjacent transfer positions. 6. An intermediate transfer member provided on a side opposite to a side on which an image of the intermediate transfer member is transferred, and electrostatically transferring a plurality of images from the plurality of image carriers to the intermediate transfer member at each of the transfer positions. The image forming apparatus according to claim 1, further comprising a plurality of transfer units for transferring. 7. The image forming apparatus according to claim 6, wherein substantially the same voltage is applied to each of said transfer means during image transfer. 8. The image forming apparatus according to claim 7, wherein a voltage applied to each of said transfer units is controlled at a constant voltage during image transfer. 9. The image forming apparatus according to claim 7, further comprising a plurality of power supplies for applying a voltage to each of said transfer units during image transfer. 10. The image forming apparatus according to claim 7, further comprising a single power supply for applying a voltage to each of said transfer units. 11. The time required for the intermediate transfer member to move from a position where an image on the intermediate transfer member is transferred to a recording material to a position where an image is first transferred to the intermediate transfer member is T ′.
2. The image forming apparatus according to claim 1, wherein τ ≦ T ′ is satisfied, where (sec). 12. The time required for the intermediate transfer member to move from a position where the last image is transferred to the intermediate transfer member to a position where the image on the intermediate transfer member is transferred to a recording material is represented by T ″.
2. The image forming apparatus according to claim 1, wherein τ ≦ T ″ is satisfied. 13. The image forming apparatus according to claim 1, wherein said intermediate transfer member is a belt supported by a plurality of supporting means. 14. The image according to claim 1, wherein a plurality of color images are sequentially transferred from said plurality of image carriers onto said intermediate transfer member in a superimposed manner, and said plurality of color images on said intermediate transfer member are transferred onto a recording material. Forming equipment.